For a 6 inch (DN150) butterfly valve held at 60° disc rotation, the flow coefficient (Cv) sits between roughly 506 and 605 on a soft-seated concentric body, and climbs to about 735 on a Class 300 high-performance (double-offset) body, per the Pratt Class 300 high-performance Cv table and the VALTEC resilient-seated chart [S1][S4].
The 60° point matters because it is the throttle range most modulating duty lands in: 60° disc rotation is shallow enough to keep the disc clear of the seat for long life, yet closed enough to give the positioner real authority. Engineers sizing HVAC, raw-water and light-chemical lines typically pick a valve whose fully open Cv is at least 1.4 to 1.6 times the calculated required Cv, so a 60°-set point still has margin.
What Cv Means on a Rotating-Disc Valve
Cv is defined as the US gallons per minute of water at 60°F that pass through a fully open valve with a 1 psi pressure drop across it [S1][S7]. A 6 inch resilient-seated concentric butterfly valve has a fully open (90°) Cv of about 1,450 according to the JRVAL concentric table, while a high-performance double-offset body in the same size reaches roughly 1,750 [S5]. The Pratt Class 300 data sheet, which is body-rated not generic, lists the 6 inch 90° point at 995 [S4]. That spread is normal: geometry, seat geometry and pressure class all move the number.
The 60° reading is not a fixed fraction of 90°. On the VALTEC concentric table, 60° on a 6 inch valve yields Cv 605 against the 90° value of 1,579, which is 38.3% of full [S1]. On the Pratt Class 300 sheet, 6 inch at 60° is 506 against 995 at 90°, which is 50.9% of full [S4]. The difference traces back to disc profile and seat geometry, not measurement error. For context on the broader butterfly valve family, the disc-throw relationship is non-linear across all sizes, so any valve sized at a non-90° point must use a published partial-angle curve, never linear interpolation [S1][S3].
Source-by-Source Numbers for 6 Inch at 60°
The VALTEC chart and the cncontrolvalve THINKTANK sheet report the same 6 inch concentric Cv at 60° of 605 (90° Cv 1,579), and they also agree across all sizes from 2 inch to 48 inch, suggesting a shared industry baseline for resilient-seated bodies [S1][S3]. The Engineering ToolBox butterfly coefficient table does not break out 60° explicitly, but its 70° entry for a 6 inch valve is 844, which interpolates smoothly between 60° and 80° in the VALTEC-style data [S2].
The Pratt industrial PDF for Class 300 high-performance butterfly valves is the only one in the research set that gives 6 inch at every 10° step. Its 60° value is 506, with 90° at 995 and 10° at just 24 [S4]. That 10° number is the giveaway that throttling quality at shallow angles is poor on this body type: a process loop that needs fine control below 30% of rated Cv should not pick a butterfly. The KFlo 500 series data sheet from Crispin uses the same water-at-60°F definition, which is the convention that makes all the other tables cross-comparable [S7].
Comparing the Main Butterfly Valve Types on Partial Cv
For a 6 inch (DN150) line, three constructions dominate the buyer's list, and each behaves differently at 60°. Concentric resilient-seated (wafer or lug) is the cheap default, with a 60° Cv of about 605 (38% of full) per the VALTEC table [S1]. High-performance double-offset Class 150 lifts the 60° Cv to roughly 735 in the Pratt data, against 914 at 90°, which is 80% efficient at 60° and is the usual pick for throttling service on water and light hydrocarbons [S4]. Triple-offset metal-seated bodies go further, but the cncontrolvalve sheet shows their 150LB and 300LB curves running hotter than resilient-seated types at the same angle [S3].
On a cost-and-control basis, the practical ranking for a 6 inch modulating service is: high-performance double-offset for tight shutoff and stable 30-70° gain; resilient-seated concentric for on-off or coarse throttling at the lowest price; triple-offset for high-temperature or fire-safe duty where the seat has to survive. For pure on-off isolation at full open, the resilient-seated concentric 6 inch delivers the highest Cv per dollar, with the high-performance body costing roughly 2 to 3 times more for only 20-25% more Cv at the same size [S5][S6]. A useful cross-reference is the butterfly vs globe control valve rangeability comparison, which lines butterfly's typical 50:1 rangeability against globe's 100:1+ in modulating duty.
Sizing Procedure at 60° Set Point
Calculate the required Cv from the standard liquid equation Q = Cv × √(ΔP / G), where Q is in GPM, ΔP in psi and G is specific gravity (water = 1.0) [S3]. For a 6 inch line delivering 600 GPM of water with 4 psi allowable drop, required Cv = 600 / √4 = 300. The valve must be sized so that its 60°-set Cv is at least 1.25 times the required Cv, which is 375 in this example. The resilient-seated 6 inch at 60° has Cv 605, so it passes with 60% authority at the set point [S1].
Engineers routinely apply the 1.25 authority rule when the disc will sit at a fixed partial angle; if the valve modulates between 40° and 70°, the worst-case Cv on the curve must still clear required Cv. The Engineering ToolBox max-flow-velocity column for a 6 inch line lists 75 ft/s at ANSI 150 and 80 ft/s at ANSI 300, which is the upper process-side limit, not the Cv limit [S2]. Always cross-check Cv against the seat's published disc-angle curve, because two valves with the same fully open Cv can differ by 15% at 60° depending on disc profile [S5].
Limits, Failure Modes and When Not to Use a Butterfly at 60°
A butterfly at 60° is operating in its mid-stroke: disc-induced turbulence is highest in this band, and cavitation risk on liquid service rises sharply above 80% of the valve's max-velocity limit [S2]. For slurry, fibrous or high-solids service, the 60° disc position exposes the seat edge to wear, and resilient-seated bodies are not the right pick. The Pratt Class 300 sheet shows 6 inch at 10° at only 24 Cv, roughly 2.4% of the fully open value, which is a useful reality check: a butterfly that looks closed on the indicator can still pass measurable flow if the seat is worn [S4].
For throttling below 20% of rated Cv, switch to a globe or ball style that has stable gain at low lift. The ball valve typically delivers 5-10 times the Cv of an equivalent-size butterfly when fully open, but a V-port ball gives the linear low-flow control a butterfly cannot match. The cncontrolvalve sheet on resilient-seated bodies includes a 2 inch at 10° value of 0.1, which quantifies the same near-closed leakage on a smaller body and confirms that butterfly shutoff at shallow angles is geometry-limited, not seat-limited [S3].
Standards, Conventions and Cross-Checking
All tables in this article use the ISA-style definition: Cv in US GPM of water at 60°F with 1 psi drop [S1][S7]. The metric equivalent Kv, in m³/h of water at 20°C with 1 bar drop, converts as Cv = 1.156 × Kv and Kv = 0.865 × Cv per the JRVAL reference [S5]. For double-checking against manufacturer data, the KFlo 500 series data sheet and the Pratt Class 300 PDF are the two most rigorous sources in the research set, because both quote Cv at every 10° step rather than just at 90° [S4][S7].
Use the manufacturer-published Cv-vs-angle curve for the specific model on the PID, not the generic table, because disc profile, seat geometry and pressure class move the 60° value by 15-20%. For a 6 inch Class 150 high-performance service, expect a 60° Cv of 700-800; for a 6 inch Class 300 high-performance service, expect 500-550; for a 6 inch resilient-seated concentric, expect 580-620 on a wafer body and slightly less on a lug body of the same nominal size [S1][S4][S5]. Always confirm the body rating matches the line class, since the same DN150 disc in a Class 300 shell has lower Cv at every angle than the same disc in a Class 150 shell, per the Pratt table [S4].
Track two signals when a 6 inch butterfly is set to hold 60° in service: actual ΔP across the disc (should stay below the value used in sizing) and seat-edge noise (cavitation starts audibly before the Cv drops visibly). If ΔP at 60° open exceeds 60% of the inlet pressure, the valve is choking and a larger size or different style is the right fix [S3].